Iliac angle as a marker for Down syndrome in second-trimester fetuses: CT measurement

To determine if the iliac angle is greater in second-trimester fetuses with trisomy 21 than in euploid fetuses and to establish the best level and plane for measuring this angle by using three-dimensional computed tomography (CT).

CT was performed in 18 formalin-preserved fetuses (eight trisomy 21, 10 euploid control fetuses), and the pelvic bone anatomy was reconstructed three-dimensionally. Iliac angles were measured in axial views at three levels in two planes. Data were analyzed nonparametrically with the Mann-Whitney test.

The mean gestational ages for trisomy 21 and control fetuses were 17.0 and 16.7 weeks, respectively. The external plane was the easiest to measure and the most reproducible. The mean iliac angles were significantly greater (P < .05) in the trisomy 21 fetuses than in the control fetuses and were as follows: superior level, 95.6 degrees vs 76.4 degrees; middle level, 84.5 degrees vs 62.5 degrees; and lower level, 78.1 degrees vs 57.5 degrees. With a 90 degrees threshold, the superior iliac angle measurement had a sensitivity of 75%, a specificity of 89%, and an odds ratio of 24 for Down syndrome.

Second-trimester fetuses with trisomy 21 have a significantly greater iliac angle than euploid fetuses have. The iliac angle varies with the axial level, with the widest angle at the most superior level. Measurement of the iliac angle at the most superior level is supported as a potential marker for Down syndrome at prenatal ultrasonography.